The present invention relates to circularly polarized broadcasting antennas, and more particularly to a broadcasting antenna comprised of plural, crossed dipoles.
Until recently, television broadcasting in the United States has almost exclusively used horizontally polarized antennas. The horizontally polarized antenna which has become most popular for VHF television broadcasting purposes is the superturnstile, or batwing, antenna. Batwing antennas provide very low windload and have extremely good electrical performance. Nonetheless, much effort has recently been directed toward devising a circularly polarized (CP) antenna which could replace existing batwing antennas. This interest in CP antennas is largely due to a recognition that reception problems associated with the use of linearly polarized broadcasting antennas and receiving antennas, such as television image "ghosting", canyon effects, and multipath interference, could be substantially avoided if CP antennas were instead used.
In order to replace a batwing-type broadcasting antenna with a circularly polarized antenna, however, a CP antenna must be designed which will not only provide good electrical performance, but will also provide tower loading (weight and windload) which is not significantly greater than that of the antenna being replaced. If this is not the case, the entire tower might also have to be replaced in order to support the new antenna; the replacement cost may then be prohibitively high.
An antenna is described herein which may serve as a direct replacement for batwing antennas currently in use, and which provides very good electrical performance. In addition to a low windload factor, this antenna additionally has good axial ratio in both the vertical and horizontal patterns as well as good aperture efficiency.
In accordance with the present invention, a circularly polarized antenna is provided wherein at least one antenna bay is provided on an antenna mast. This antenna bay is comprised of three individual circularly polarized antennas, equally spaced circumferentially about the mast. Each individual circularly polarized antenna is comprised of a pair of crossed dipoles having drooping arms. The angle at which the arms droop is selected so that good axial ratio is achieved at all points in both the vertical and horizontal patterns. Three reflectors are provided which extend radially from the mast at circumferential positions intermediate the antennas so as to substantially isolate the radiation patterns provided by the three circularly polarized antennas.
Also in accordance with the present invention, several bays of this type are provided at axially spaced positions along a common mast, where the axial spacing is selected to provide optimal antenna gain. An antenna having high aperture efficiency is thereby provided.